Capacitance change detection devices are used for detecting a trigger for opening operation (i.e., unlock operation) of systems for controlling opening and closing of a vehicle door for such as automobiles. More particularly, when a mode of a device provided at a vehicle is switched to an unlock allowance mode because a user approaches the vehicle and a code is identified between the device provided at the vehicle and a remote controller carried by the user, and when the user further touches an unlock sensor (i.e., an electrode) provided inside of an outside handle of the vehicle door, the device provided at the vehicle detects a change of capacitance of the electrode of the unlock sensor and carries out the unlock operation. That is, the capacitance change detection devices detect an intention of a user to unlock a vehicle door as an output of an unlock sensor (i.e., a change of capacitance).
A known capacitance change detection device which generates either an electric field or an electromagnetic field around an antenna by means of an electronic circuit and the antenna connected to the electronic circuit for detecting a very little change of capacitance on the basis of a change of either the electric field or the electromagnetic field around the antenna is described in U.S. Pat. No. 5,764,145. The known capacitance change detection device described in U.S. Pat. No. 5,764,145 is configured to avoid an influence of temperature and humidity upon a change of capacitance by passing a signal from an external electric field and an electromagnetic field received via an antenna through a highpass filter.
According to the known capacitance change detection device described in U.S. Pat. No. 5,764,145, a change of capacitance because of a change of temperature and a change of humidity are excluded from objects to be detected by providing a highpass filter. In other words, with the construction of the known capacitance change detection device described in U.S. Pat. No. 5,764,145, a sensitivity regarding variations is not changed.
In U.S. patent application Ser. No. 10/951883, the applicant provides another capacitance change detection device. According to the capacitance change detection device described in U.S. patent application Ser. No. 10/951883, by repeatedly charging a pre-discharged known reference capacitance by an unknown capacitance to be measured, and the number of charging until an electric voltage at both ends of the reference capacitance reaches a predetermined electric voltage is counted. When the number of charging is reduced by equal to or greater than a predetermined threshold value, it is detected as an increase of unknown capacitance to be measured.
Further, according to the capacitance change detection device described in U.S. patent application Ser. No. 10/951883, because capacitance to be measured is changed in accordance with a change of surrounding environment (e.g., capacitance at wire harness which is connected to the capacitance to be measured, or the like, is changed because of a change of an environment), the number of charging until the electric voltage at the both ends of the reference capacitance reaches the predetermined electric voltage is varied depending on the environment. Thus, when judgment of the number of charging is performed with the same threshold value at around different environments, the judgment results may be differed from each other even when variations of the capacitance to be measured is identical. In other words, there is a drawback that sensitivity regarding variations of capacitance to be measured depends on the environment.
A need thus exists for a capacitance change detection device which detects variations of capacitance to be measured on the basis of a change of a correlation value of capacitance to be measured which is measured on the basis of a reference capacitance.